Update ADK doc according to issue #1113 - 3 - EnterpriseWebSearchTool for Vertex AI Search

[adk-bot]

3. Clarification on Search Tools
   The EnterpriseWebSearchTool is for Vertex AI Search, which is different from Google Search grounding (google_search tool). The documentation can be confusing.

   Proposed Change:
   Add a new document or a section in the tools documentation to clarify the differences between the various search tools available in ADK.

   Current state:
   The documentation primarily focuses on Google Search grounding and does not clearly differentiate it from other search offerings.

   Proposed Change:
   Create a new page or section that explains:

   • Google Search grounding (google_search tool): For Gemini models.
   • Enterprise Web Search (EnterpriseWebSearchTool): For Vertex AI Search.
   • How to choose the right tool for their use case.

   Reasoning:
   The distinction between the different search tools is not clear in the current documentation, which can lead to confusion.

   Reference:

   • src/google/adk/tools/enterprise_search_tool.py